meson: Make gst-codecparser optional

[philipl]

Disable vp8 and vp9 if not available.

I'm not sure how truly useful this is, but gstreamer has a huge pile of dependencies, and maybe one might not want to install all of them just to get access to the codecparser if vp8 and vp9 aren't essential for you.

[elFarto]

Thanks for the PR. A couple of things:

• VP8 doesn't use gst-codecparsers any more, so it doesn't need to be disabled.
• I don't think we need the #ifdefs to remove the checks, as there's already code below that to filter the list of hardware supported codecs with whatever ones we have compiled support for.

[philipl]

Thanks for the review. I've updated accordingly.

[elFarto]

Looks good!